Rosetta To Land on Comet

On February 26th, at 04:36 a.m. Kourou time (07:36 a.m. GMT – 08:36 a.m. CET), Rosetta is scheduled to be launched on board an Ariane-5 launcher from the European Spaceport in Kourou, French Guiana. The Rosetta spacecraft will be pairing up with Comet 67P/Churyumov-Gerasimenko and accompanying it on its journey, investigating the comet’s composition and the dynamic processes at work as it flies sunwards.

Rosetta lander on comet surface Credit: ESA

The spacecraft will even deposit a lander on the comet. "This will be our first direct contact with the surface of a comet," said Dr Manfred Warhaut, Operations Manager for the Rosetta mission at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany.

The trip is certainly not short: Rosetta will need ten years just to reach the comet. This places extreme demands on its hardware; when the probe meets up with the comet, all instruments must be fully operational, especially since it will have been in ‘hibernation’ for two and a half years of its journey. During this ‘big sleep’, all systems, scientific instruments included, are turned off. Only the on-board computer remains active.

Originally timed to begin about a year ago, Rosetta’s journey had to be postponed. This delay meant that the original mission’s target, Comet Wirtanen, could no longer be reached. Instead, a new target has been selected, Comet Churyumov-Gerasimenko, which Rosetta will encounter in 2014 after a "billiard ball" journey through the Solar System lasting more than ten years. This will be the first mission to orbit and land on a comet.

Rosetta’s hardware fits into a sort of aluminium box measuring just 12 cubic meters. The scientific payload is mounted in the upper part, while the subsystems – on-board computer, transmitter and propulsion system – are housed below. The lander is fixed to the opposite side of the probe from the steerable antenna. As the spacecraft orbits the comet, the scientific instruments will at all times be pointed towards its surface; the antenna and solar panels will point towards the Earth and Sun respectively.

Illustration of Rosetta sitting on comet surface Credit: ESA

For trajectory and attitude control and for the major braking maneuvers, Rosetta is equipped with 24 thrusters each delivering 10 Newtons (units of force). That corresponds to the force needed here on Earth to hold a bag containing 10 apples. Rosetta sets off with 1650 kg of propellant on board, accounting for more than half its mass at lift-off.

Just 20% of total mass is available for scientific purposes. So when developing the research instruments the same rule applied as for supermodels: make every gram count. The calculation seems to have worked out right: the main probe will be carrying 11 scientific instruments and the Rosetta lander a further ten. They will analyze the composition and structure of the comet’s nucleus and study its interaction with the solar wind and the interplanetary plasma.

"To provide the probe with the power it needs in space, we have given it the biggest solar panels ever carried by a European satellite," Manfred Warhaut explained. "These cells are its only source of electricity." They span 32 meters tip to tip while, at 64 square meters the surface area is comparable to that of a two-bedroom apartment. The panels may be rotated through 180° to catch the maximum amount of sunlight.

During the comet flyby, the closest view yet of a comet at about 149 miles. Credit: NASA/JPL

These dimensions are also essential because when Rosetta meets Churyumov-Gerasimenko it will be 675 million kilometers (around four hundred million miles) away from the Sun. At that distance solar radiation is very weak and the solar collectors will supply only 440 W of power – compared with 8000 W towards the end of the mission when the two companions come closest to the Sun (at some 150 million kilometers (90 million miles) from our star distance). "The probe is also equipped with a set of four 10-amp-hour batteries to maintain power supply while Rosetta flies in the shadow of the comet."

The Rosetta lander is another of the mission’s technical highlights. Using its scientific instruments, its job will be to investigate the comet’s surface on location. Thanks to a mechanical arm, the lander will operate in a two-meter radius.

The soft landing is a particular problem given the extremely weak gravitational force exerted by the very small comet nucleus; the lander, weighing in at 100 kg on Earth, will on the comet be as light as a sheet of paper. If there were the slightest recoil, it would bounce back uncontrollably like a rubber ball. To make sure this doesn’t happen, the lander’s three legs are equipped with special shock-absorbers which take up most of the kinetic energy. The legs are also fitted with ice pitons; these bore into the ground immediately on touchdown.

At the same moment, the lander fires a harpoon to anchor it to the ground – an opportunity also to investigate the mechanical properties of the surface. "If everything goes according to plan, the mission results could well fundamentally expand our knowledge of comets, just as the Rosetta Stone, after which the probe is named, helped unravel the mystery of Egyptian hieroglyphics," said Manfred Warhaut.

En route to the comet, Rosetta will inspect two asteroids (Otawara and Siwa) at close quarters. In 1986, ESA’s Giotto spacecraft performed the closest comet fly-by ever achieved by any spacecraft (at a distance of 600 kilometers from Halley). It sent back wonderful pictures and data that showed that comets contain complex organic molecules. These kinds of compounds are rich in carbon, hydrogen, oxygen, and nitrogen. Intriguingly, these are the elements which make up nucleic acids and amino acids, which are essential ingredients for life as we know it. Giotto continued its successful journey and flew within about 200 km of Comet Grigg-Skjellerup in 1992.

Other past missions that have flown by a comet are: NASA’s ICE mission in 1985, the two Russian Vega spacecraft and the two Japanese spacecraft Suisei and Sakigake that were part of the armada that visited Comet Halley in 1986; NASA’s Deep Space 1 flew by Comet Borelly in 2001 and NASA’s Stardust, which flew by Comet Wild 2 earlier in January and has captured samples of the comet’s coma to be returned in 2006. Unfortunately NASA’s Contour launched in summer 2002 failed when it was inserted into its interplanetary trajectory. Later this year will also hail the launch of Deep Impact, a spacecraft that will shoot a massive block of copper into a comet’s nucleus.